Also, a lot of UFC fighters cut ridiculous amounts of weight, which is not usually the case in Japan. Take Thiago Alves or Anthony Johnson as extreme examples of the "American way." Then, take a look at Nakamura, a very flabby 205-er.
Look at the difference in physique between say, a Tyson Griffin or Sean Sherk at 155, and Caol Uno as additional examples.
Also, tbere's the issue of reach. Japanese fighters, on average, have a shorter reach, at the same height as other fighters.
In effect, Japanese fighters are often fighting a full weight class higher than they should be once they reach the UFC. Akiyama is the most recent example of this.